Pectin is a natural material that is abundantly present in plants, and thus is a major part of a typical human's diet. Pectin may be isolated from appropriate plant material by aqueous extraction, or other known methods. About 50,000 MT/year of pectin is commercially sold, mostly for use as an ingredient in industrially prepared foods.
Pectin generally is a water-soluble mixture of macromolecules with distinctly different macromolecular parts that can be present in different amounts. The main component of pectin is polymerized anhydrogalacturonic acid that has some of its carboxyl groups esterified with methanol. The percentage of the carboxyl groups that are methyl esterified is referred to as the degree of (methyl) esterification (DM).
Commercial pectin preparations are classified into HM-pectin (high methyl ester pectin) and LM-pectin (low methyl ester pectin) according to whether the DM is above or below 50. Pectin, in some instances, can include amidation and/or acetate esterification. The degree of amidation (DA) and the degree of acetate esterification D(Ac) are the numbers of amide groups and acetate ester groups, respectively, per hundred anhydrogalacturonic acid repeating units. Amidation usually only is significant in pectin samples that have been exposed to ammonia during their manufacturing.
Acidified Milk Drink (AMD) is a generic term that includes fluid products that contain protein of milk origin and are at least slightly acidic so that the proteins are prevented or inhibited from forming the suspension present in natural milk. Typically, in this context, “fluid” implies that the products are suitable for drinking. Fermented milk drinks, which include drinkable yogurt, are one type of AMDs. Fermented milk drinks can, in certain instances, be prepared from natural milk by fermentation with a bacterial culture so that the product attains an acidic pH, such as, for example, less than 4.4.
Other bacterial cultures may also be used for making similar fermented milk products like kefir and the Danish product Cultura. The lowered, i.e., acidic, pH of AMD may also be accomplished by using either fruit juices or food-compatible acids as ingredients together with the milk.
Typically, in natural milk that has not been acidified, protein exists as suspended bodies that are so small that they cannot be detected as individual bodies by ordinary vision; nor can natural milk be distinguished from a homogeneous liquid by the senses of the oral cavity. Natural milk, however, is white and opaque because the suspended bodies are large enough to disperse visible light. Under normal conditions in fresh natural milk, the protein bodies repel each other so that they do not aggregate into larger lumps. Upon lowering pH, however, the suspended protein bodies can lose their mutual repulsion, and aggregate. In certain conditions, this may lead to a gel with a network of aggregated protein particles, such as, for example, ordinary yogurt. Ordinary yogurt typically is eaten with a spoon.
In some instances, yogurt is reasonably stable during its normal shelf life. The signs of instability that may be observed, such as, for example, a small or moderate amount of whey exudation, are traditional and generally accepted by consumers. In contrast, if the curd is ruptured for making a fluid drinkable product, the aggregation tends to continue. If it does, the product may segregate into two or more phases that may appear notably different either by the way they look, or by how they feel in the mouth. For example, the protein may, if it is poured into a container, form visible lumps on the walls of the container. Typically, these traits are unappealing to the consumer. In some instances, another symptom of instability can be excessively high viscosity. The desired viscosity, in certain instances, depends upon customer preferences.
Pectin is one stabilizing additive that has been used to prevent the aggregation of suspended proteins in AMDs, thereby preventing the suspension from segregating into larger individual phases. It is believed that the pectin adsorbs to the sticky surfaces of the protein bodies, and the new surface (of adsorbed pectin molecules) is, in contrast to the old one, non-sticky. Other water-soluble polymers can stabilize in a similar way, for example, carboxy methyl cellulose, propylene glycol alginate, and soybean fiber.
In many current applications, pectin of fairly high DM is preferred. Not wishing to be bound by any particular theory, it is believed that segments in the pectin molecule with locally high presence of negatively charged unesterified carboxyl groups bind to the protein surfaces. The other parts of the pectin molecule are believed to create the hydrated non-sticky layer, because they, on one hand, are connected to the protein-binding pectin segments, but, on the other hand, they themselves possess more affinity to the serum phase and relatively less affinity for the protein surface. It is believed that pectin of fairly high DM works particularly well because it possesses the appropriate balance between molecular segments that adsorb to acidified protein and molecular segments with affinity for the serum. Notwithstanding the possible explanations, it remains a fact that commonly used pectins like YM-115, YM-150, and JMJ (CP Kelco), AMD 382 and AMD 453 (Danisco), and AYD 240 (Cargill) possess DM above 60.
The older published literature about AMDs (see, e.g., Glahn, P. E., PROG. FD. NUTR. SCI. 6, 1982, 171; and Glahn, P. E., et al. FOOD INGRED. EUR. CONF. PROC. 1994) describes processes in which the AMD manufacturers start with milk, either natural milk, or a dispersion of powdered milk in water, which is initially heated to eliminate microorganisms, then cooled to a temperature suitable for fermentation. The milk is then fermented after inoculation with a bacterial culture. When the fermentation process reaches the desired acidity, a solution of pectin is added to the milk, which is then stirred and homogenized. Therefore, the pectin is added after fermentation.